There are a few main reasons why people struggle with Chin Ups. In most cases, it's either because they are too heavy, or too weak, or both. Look at NFL players, three-hundred pound lineman average six to ten bodyweight chin-ups if they're good. One-hundred and eighty pound cornerbacks will average twenty + chin ups when tested. The lineman are all much stronger than these cornerbacks, but they are also much heavier. Bigger bodies don't have the same control that smaller bodies do. The line man can do chin ups at that weight because they are all "scary strong". The cornerbacks are also very strong, but they are lighter and that's why they can do more reps than their bigger counterparts.
Dropping excess body-fat and lifting heavier to get stronger would be the ideal situation to start doing chin ups. There is no ideal weight when it comes to being able to pull your mass, everyone will vary greatly. But as you get lighter/leaner, you will be able to become more successful with all bodyweight movements (push ups, chin-ups, lunges, etc).
Exercises like Lat Pull Downs, Low Rows, Squats, Dead Lifts, Sled Pushes, Military Presses, Bench Presses and Heavy Carries will all help to make your body stronger overall. As your strength goes up, the likelihood of you being able to do a chin-up goes up too. Strong is relative, it can mean different things, but if you can't do a proper push up or a proper chin up, you're probably not that strong, and/or you're too heavy.
Another thing to think about during your training is your moment arms. A Moment Arm is the distance between the joint and the resistance. So simply put, if you have longer arms or longer legs you have to travel a farther distance to complete a rep. A person that is 6'6 has a much longer distance to travel during a dead lift than a person that is 5'6. They both may be very strong, but the longer person will have to overcome that moment arm length. A person that has shorter arms may be better at chin ups or bench pressing over a person with longer arms because of the moment arm. This isn't a good or bad thing, it's just simple physics and something to think about.
But there's good news! With a lot of clean eating and a ton of strength training, you can get to your goal of doing a Chin Up!
Side Note: A Chin-up is performed with a supinated grip (palms towards you). This allows for more synergists (helper muscles) to kick in and help out the lats when pulling. A Pull-up is performed with a pronated grip (palms away from), this is usally harder for most people becuase it increases the moment arm distance.
There are a ton of ways to train your body to do chin-ups:
Superbands around your knee (for assistance)
Superbands around your foot (more assistance)
Partner Assistance (coach pushes your lats while you pull up)
Eccentric Drops (video)
The "Jump Up and Drop Slow" eccentric version is possibly my favorite. It allows your entire body to get tight and have a fight with gravity. I warn you now though, these will cause excessive back and ab soreness (in a good way). I would start with 3-4 sets of 3-4 five-second drops. Make sure you completely extend your arms at the bottom. Fight the fall for the entire length of the movement. After a few weeks of hammering away at these, chin-ups will start to become a reality.
So what do we need to work on to be able to perform chin-ups?
* Get Stronger
* Get Leaner (lose excess body-fat)
* Practice them A LOT
* Don't ever give up